The Form.FormClosing and Form.FormClosed events are raised when a System.Windows.Forms.Form is closed, either through user action or programmatically. Handlers for these events receive parameters of type System.Windows.Forms.FormClosingEventArgs and System.Windows.Forms.FormClosedEventArgs, respectively. Both of these event argument classes use the System.Windows.Forms.CloseReason enumeration.
The owner form is closing.
The parent form of this multiple document interface (MDI) form is closing.
The cause of the closure was not defined or could not be determined.
The Microsoft Windows Task Manager is closing the application.
The user is closing the form through the user interface (UI), for example by clicking the Close button on the form window, selecting Close from the window's control menu, or pressing ALT+F4.
The operating system is closing all applications before shutting down.